60 THE FERTILITY OF THE SOIL [CH. 



cattle can therefore be kept and more manure can be 

 made for the arable land. 



It often happens, especially on the lighter soils, 

 that the crop supplied with phosphates is now limited 

 by the deficiency of potassium salts. This deficiency 

 has long been partially met by dressings of wood 

 ashes, salt, etc., but a better method was needed. 

 Fortunately large supplies of potassium salts were 

 discovered at Strassfiirt in Germany and were put on 

 the market in 1861. Since then they have been ex- 

 tensively used, although curiously enough no similar 

 deposits have been found elsewhere. 



The modern farmer is no longer dependent on 

 leguminous plants for his supplies of nitrogen. 

 Nitrate of soda is imported from Chili, sulphate of 

 ammonia is manufactured from coal at home, and a 

 large number of grains and seeds are imported from 

 over the seas to feed the cattle and thus increase the 

 supply of farmyard manure. 



Modern agricultural systems are far too complex 

 to reduce to any simple rigid order ; but their general 

 bearing on the fertility of the soil may be briefly 

 summed up: 



1. The supply of plant nutrients is kept up by 

 the addition of appropriate artificial manures. It is 

 iinix>8sible to determine a priori either by chemical 

 analysis or otherwise exactly what mixture of manures 

 will be best ; nothing but direct trials suffice. But a 



